Mitchell, according to head coach Kyle Shanahan, "will be out for a couple weeks" with an adductor strain, David Lombardi of The Athletic reports.
Mitchell was a surprise scratch from the roster for Saturday's preseason opener against the Chiefs, and the reason for his absence is now known. He's an intriguing prospect for the 49ers, and one who could have made a preseason push, but he's a bit down the depth chart, so it's unlikely the team's early season running back rotation will be altered significantly.